There is demand for the structural materials constituting transport equipment such as aircraft to amply satisfy certain mechanical characteristics, as well as be lighter in weight and lower in cost. Among these, the shift to FRPs as the primary structural material of components such as the wings, the tailplane, and the fuselage is being investigated in order to achieve reduced weight.
In addition, recently there has been movement toward FRPs as reduced weight in the structural materials of automobiles is being sought, and demand for cost reductions greater than that of aircraft is becoming stronger.
Autoclave molding is known as a typical production process for such FRPs.
In autoclave molding, a pre-preg is used as FRP material, the pre-preg being reinforcing fibers impregnated with a matrix resin in advance. By inserting the pre-preg into a mold in the shape of the component and then laminating, heating, and applying pressure, an FRP is formed.
A characteristic of the pre-preg used herein is that it is possible to control to a high degree the reinforcing fiber volume fraction Vf. This has the advantage of enabling an FRP with excellent mechanical characteristic to be obtained. However, the pre-preg itself is an expensive material that requires refrigeration facilities for storage, and the productivity thereof is low since an autoclave is used. Thus, the pre-preg is also problematic in that molded parts formed therefrom are high in cost.
In addition, in the case wherein the shape of a molded part is that of a C or similar shape, only out-of-plane strain of the pre-preg or a laminate of laminated pre-pregs is sought, whereas in the case wherein the shape of the molded part is spherical, partly spherical, or block-shaped, in-plane shear strain is sought in addition to out-of-plane strain. However, since the reinforcing fibers of the pre-preg are held in place by matrix resin, in-plane shear strain is essentially impossible, and thus the draping of pre-pregs into complex shapes having two-dimensional curvature is extremely difficult.
A method of improving drapability is known wherein, when drape forming a pre-preg like the above into a shape wherein in-plane shear strain is sought, restriction of the reinforcing fibers by the matrix resin is lowered by applying heat to lower the viscosity of the matrix resin. However, since reinforcing fibers in a pre-preg are typically arranged in a uniform and dense manner, the reinforcing fibers are not easily moved due to friction among reinforcing fibers, even when the viscosity of the matrix resin is lowered by heat. For this reason, even though drape formation of a shape that requires out-of-plane strain, such as a C shape, can be improved by applying heat, there is a problem in that draping form is hardly improved for shapes wherein in-plane shear strain is sought, such as a spherical surface or block shape. For this reason, when it is necessary to drape form a shape having two-dimensional curvature, it has been necessary to process the pre-preg, such as by adding precuts. However, if precuts are added, the continuity of the reinforcing fiber is lost, and there is a new problem in that elasticity and strength are lowered.
Meanwhile, resin injection molding processes such as resin transfer molding (RTM) are known to be molding processes that improve FRP productivity and reduce molding costs. In these resin injection molding processes, reinforcing fibers that have not been impregnated with matrix resin are first placed inside a mold and then matrix resin is injected thereinto, thereby impregnating the reinforcing fibers with matrix resin and forming an FRP. The matrix resin is then hardened by heating using an oven or similar equipment.
Since the resin transfer molding process uses dry reinforcing fiber base material, materials costs can be reduced. Furthermore, since an autoclave is not used, molding costs can be reduced.
Normally, in the resin transfer molding process, first a preform that maintains the shape of the final product is prepared, the preform being constructed from dry reinforcing fiber base material that has not been impregnated with matrix resin. After placing the preform inside the mold, matrix resin is injected, thereby forming an FRP.
The preform is obtained by using a mandrel or mold in the shape of the final product, wherein reinforcing fiber base material is laminated on the basis of a predetermined lamination configuration, the laminate being shaped to fit the mandrel or mold.
In the case where the preform is a C shape, essentially only out-of-plane strain is sought for the reinforcing fiber base material or the laminate made of laminated reinforcing fiber base material, whereas in the case where the preform is spherical, partly spherical, or block-shaped, in-plane shear strain is also sought.
Multi-axial woven fabrics, such as woven fabrics having fiber filaments arranged in two axial directions, are known as reinforcing fiber base materials that enable in-plane shear strain. Such woven fabrics form a reinforcing fiber base material by the intersection of reinforcing fiber filaments with each other. As long as the reinforcing fibers are not restricted by auxiliary fibers or similar means, it is possible for the angles whereby the reinforcing fibers intersect to change, thereby enabling in-plane shear strain. However, since the reinforcing fiber filaments are arranged multiaxially, the number of reinforcing fiber filaments in each direction essentially halves in the case of a biaxial woven fabric, for example. Thus, while drapability is excellent compared to unidirectional reinforcing fiber base material, there is a problem in that mechanical characteristics are poor.
In addition, a method is known whereby, in order for the preform made from the reinforcing fiber base material to maintain the shape of the final product or a shape close to that of the final product, the reinforcing fiber base material is laminated and draped form in a mandrel or mold having the final shape. Subsequently, the adhesive properties of thermosetting resin or thermoplastic resin are used to unify the reinforcing fiber base material and preserve the preform shape.
For example, a method has been proposed wherein an adhesive agent that contains a thermosetting resin is adhesed to a reinforcing fiber base material, and after laminating the reinforcing fiber base material on the basis of a predetermined lamination configuration, ample pressure is applied to the laminate, thereby obtaining an FRP using a preform that can maintain product shape even after pressure release (cf. Patent Literature 1).
However, according to the above proposal, the laminate of reinforcing fiber base material is compressed with sufficient pressure to maintain the product shape even after pressure release, and for this reason it is extremely difficult to deform the laminate after applying pressure. For this reason, it is necessary to prepare the preform by applying pressure after first adjusting the shape of the reinforcing fiber base material by draping form in a mold or similar means in the shape of the product. However, in such a method, it is necessary to laminate the reinforcing fiber base material one layer at a time, particularly when draping form the reinforcing fiber base material into a complex shape. For this reason, there is a problem in that the draping form process takes time. Moreover, when trying to drape form a non-unified multi-layer laminate in a mold having a complex shape, trouble can occur, such as the reinforcing fiber base material unraveling during draping form, and thus handling is problematic.
To counter this problem in draping form reinforcing fiber base material into complex shapes, methods have been proposed wherein, for example, an arbitrarily shaped preform is shaped by hanging reinforcing fibers on a large number of parallel pins (cf. Patent Literature 2). In this method, the reinforcing fibers are arranged in a predetermined laminate structure by adjusting the positions of the pins whereon the reinforcing fibers are hung. In addition, a preform of arbitrary width can be obtained by adjusting the distance between pins.
However, when this method is used for members having both considerable thickness and wide surface area, such as structural material for aircraft, it is necessary to arrange a large number of pins and additionally to hang reinforcing fibers many times on the pins. For this reason, there is a problem in that the method requires an inordinate amount of work and time.
In addition, a method has been proposed wherein an FRP is formed using a preform bonded in the direction of the thickness of the reinforcing fiber base material by arranging fibers in the direction of thickness of a laminate formed by laminating reinforcing fiber base material of biaxial woven fabric (cf. Patent Literature 3). In this method, by arranging fibers in the direction of thickness at the portions where strain is not required without arranging fibers in the direction of thickness at the portions where strain is required, drapability is ensured while improving handling. However, in this method, a biaxial woven fabric is used. In a biaxial woven fabric, reinforcing fibers are woven in two directions, and as such the reinforcing fiber count in each direction essentially halved. Moreover, since the vertical fibers and the horizontal fibers have nearly the same fineness, a large amount of crimping in the reinforcing fibers occurs at the intersection points of vertical and horizontal fibers due to fiber bending. As a result, there is a trouble in that the realized mechanical characteristics are approximately only half that of a pre-preg wherein reinforcing fibers are arranged in a unidirectional manner.
Since extremely high mechanical characteristic are demanded of the primary structural material for aircraft in particular, biaxial woven fabric, while excellent in drapability and handling, is problematic in that the mechanical characteristics thereof are insufficient.
This being the case, a unidirectional reinforcing fiber base material combining drapability, mechanical characteristics, and handling, as well as a laminate made by laminating and unifying a plurality of layers of such reinforcing fiber base material, and a preform and FRP made from the same, have not been obtained, and there is a need for technology that satisfies these demands.    Patent Literature 1: Japanese patent application publication (Translation of PCT Application) No. H9-508082    Patent Literature 2: Japanese patent application Kokai publication No. 2004-218133    Patent Literature 3: Japanese patent application Kokai publication No. 2004-36055